1. Field of the Invention
The present invention relates to an optical disk device, and in particular, to optimization of focus offset when data is written onto a recordable optical disk.
2. Description of the Related Art
Recordable or rewritable optical disk devices such as DVD-RAMs and the like have been developed and are being commercialized.
In order to write data accurately onto such recordable or rewritable optical disks, writing power must be optimized and, to optimize power, the focus offset for writing must be optimized. To this end, the optimal focus offset for replaying is used as the focus offset for writing. Alternatively, as disclosed in Japanese Patent Laid-Open Publication No. 2001-52351, the optimal focus offset for writing may be determined by varying the focus offset while writing data and then evaluating the replay quality.
However, because the optimal focus offset for replaying may not necessarily be identical to the optimal focus offset for writing, there had been a problem in that a high-grade writing may not be possible even when the optimal focus offset for replaying is used when writing.
FIG. 15 shows changes in RF level and jitter for replay signals when data is replayed while the focus offset is varied. As shown, the jitter is minimized and the RF level is maximized at the optimal focus offset For. When the focus offset is varied from the optimal value when the objective lens for optical pickup is moved towards or away from the optical disk, jitter and RF level are degraded.
FIG. 16 shows changes in RF level and jitter when data are written while the focus offset is varied and then replayed at the optimal focus offset For as shown in FIG. 15. At the optimal focus offset Fow, the jitter is minimized and the RF level is maximized. When the focus offset is varied by moving the objective lens towards or away from the optical disk and data is written, jitter and RF level are degraded.
FIG. 17 shows the relationship between the optimal focus offset For for replaying as shown in FIG. 15 and the optimal focus offset Fow for writing as shown in FIG. 16. As is clear from FIG. 17, the optimal focus offset For for replaying does not coincide with the optimal focus offset Fow for writing and there is a difference between the two focus offsets. Thus, even when data is written using the optimal focus offset For for replaying, for example, the jitter may not necessarily be minimized and the error rate or the like may increase.
On the other hand, this problem can be solved by determining the optimal focus offset Fow based on the characteristics shown in FIG. 16 and setting the offset of the optical disk device to Fow. However, the optimal focus offset Fow is determined for a reference optical disk, and is not necessarily an optimal value for each optical disk on which data is to be written. In particular, because a particular individual may select any disk from among the many optical disks are available on the market, writing at a fixed focus offset may sometimes not be appropriate. While it is possible to determine an optimal focus offset for writing every time a new optical disk is loaded, this requires that data be written while the focus offset is varied and that the writing power be optimized through separate optimization processes: this, in turn, requires relatively long processing time.
Accordingly, an object of the present invention is to provide a device in which data can be consistently written onto an optical disk at an optimal focus offset.
According to one aspect of the present invention, there is provided an optical disk device comprising means for writing a mark onto an optical disk at a predetermined focus offset; means for replaying the mark while varying the focus offset; means for detecting a change in the quality of a replay signal with respect to the variation of the focus offset; and means for adjusting the predetermined focus offset based on the change in the quality of the replay signal. The means for writing writes data onto the optical disk at the focus offset adjusted by the means for adjusting. There is a relationship between the optimal focus offset for writing and changes in the quality of replay signals with respect to variations in the focus offset when replaying, and this relationship is approximately a constant and independent of optical disks. More specifically, although the optimal focus offset for writing differs for each optical disk, the relationship between the optimal focus offset for writing and changes in the quality of the replay signal is approximately constant and is independent of the optical disks. According to the present invention, this relationship is employed for obtaining the optimal focus offset for writing for each optical disk.
According to another aspect of the present invention, it is preferable that a change in the error rate with respect to the variation of the focus offset is used as the change in the quality of the replay signal with respect to the variation of the focus offset. As the change in the quality of the replay signal with respect to the variation of the focus offset, a change in jitter with respect to the variation of the focus offset or a change in RF level with respect to the variation of the focus offset may also be preferably used.
According to yet another aspect of the present invention, the optical disk device further comprises means for storing the relationship, for a reference optical disk, between the predetermined focus offset and the change in the error rate. The means for adjusting adjusts the predetermined focus offset based on the relationship, for the reference optical disk, between the predetermined focus offset and the change in the error rate, and a change in the error rate, in the optical disk, with respect to the variation of the focus offset and sets as the optimal focus offset for the optical disk onto which data is to be written. The means for storing may also store the relationship, for a reference optical disk, between the predetermined focus offset and the change in jitter or may store the relationship, for a reference optical disk, between the predetermined focus offset and the change in the RF level.
The present invention can be more readily understood by referring to the following embodiments. The embodiments, however, are presented as examples only, and are not intended in any way to limit the scope of the present invention.